Current limiting device

ABSTRACT

The current limiter comprises two spaced opposite electrodes interconnected into a unitary structure by an insulation. A compartment formed within the electrodes and insulation is hermetically closed by a plug screw threaded into one of the electrodes and provided with a hole forming a part of the compartment. Disposed within the compartment are a current limiting material normally put in a liquid or solid state and a pressure relief member. The material responds to a short circuit current to be evaporated to limit the current after which it returns to its original state.

[56] References Cited UNITED STATES PATENTS 3,501,730 3/1970 Ito et337/114 X Primary ExaminerBernard A. Gilheany Assistant Examiner-DewittM. Morgan AttorneysRobert E. Burns and Emmanuel J. Lobato ABSTRACT: Thecurrent limiter comprises two spaced opposite electrodes interconnectedinto a unitary structure by an insulation. A compartment formed withinthe electrodes and insulation is hermetically closed by a plug screwthreaded into one of the electrodes and provided with a hole forming apart of the compartment. Disposed within the compartment are a currentlimiting material normally put in a liquid or solid state and a pressurerelief member. The material responds to a short circuit current to beevaporated to limit the current after which it returns to its originalstate.

Inventor Kiyoshi Yamaqata Fukuyama, Japan Appl. No. 24,321 [22] FiledMar. 31, 1970 Patented Oct. 5, 1971 Assignee Mitsubishi Denki KabushikiKaisha Tokyo, Japan Apr. 2, 1969 Japan 44/25374 2 Claims, 1 Drawing Fig.

[52] 0.8. 337/114, 337/21 HOlh 85/06,

HOlh 87/00 337/21, 114, 119, 159, 290

United States Patent 32 Priority [54] CURRENT LIMITING DEVICE s01FieldofSearch...............................

CURRENT LIMITING DEVICE BACKGROUND OF THE INVENTION This inventionrelates to improvements in a current limiting device of self-restorationtype capable of repeatedly limiting extraordinary currenm such asshort-circuit current apt to cause faults in wiring and equipments of anelectric circuit as do overload currents.

Current limiting devices of the type referred to may sometimes be called"permanent fuses" and include the current limiting material composed ofa low melting point metal liquidized or solidified at room temperatureor at relatively low temperatures up to 100 C. at the outside. Examplesof such low melting point metals involve sodium (Na), potassium (K),sodium-potassium (NaK), mercury (Hg) etc. The current limiting materialas above described electrically good conductive and is responsive to ahigh current flowing therethrough to generate Joule heat therein to beevaporated thereby to form a plasma very high in vapor pressure. Theplasma exhibits its specific resistance greatly higher than the shortcircuit impedance of the associated circuit thereby to limit theparticular short circuit to a predetermined magnitude or less. inaddition after the completion of the current limiting operation, theevaporated material is rapidly cooled to be liquified or solidifiedstate. That is, the material is self-restored to the originalelectrically good conductive material.

In the conventional current limiting devices of the type referred tothere has been a fear that, upon evaporating the current limitingmaterial, the enclosed compartment filled with that material hasabruptly increased in pressure thereby to release the screw threadedscrew for the compartment.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention toeliminate the above-mentioned disadvantage of the prior art practice.

The invention accomplishes this object by the provision of a currentlimiting device comprising, in combination, a pair of electrodesdisposed in spaced parallel relationship, an electric insulation forinsulating interconnecting the electrodes into a unitary structure, acompartment formed within the electrodes and insulation, a screwthreaded plug screw threaded in one of the electrodes to hermeticallyseal the compartment, and provided with a hole forming a part of thecompartment, and a current limiting material filling the compartment andresponsive to a current in excess of a predetermined magnitude flowingthrough the device to be evaporated to perform a current limitingoperation after which the material is selfrestored.

BRIEF DESCRIPTION OF THE DRAWING The invention will become more readilyapparent from the following detailed description taken in conjunctionwith the accompanying drawing wherein a single FIGURE is an axialsectional view, partly in elevation, of a current limiting deviceconstructed in accordance with the principles of the inventio with partsbroken away.

DESCRlPT ION OF THE PREFERRED EMBODIMENT Referring now to the drawing,it is seen that an arrangement disclosed herein comprises a pair ofelectrodes and 12 disposed in spaced opposite relationship and anelectric insulation 14 for mechanically interconnecting the electrodes10 and 12 therewith into a unitary structure while surrounding the outerperipheries of both electrodes. One of the electrodes, in this case, thelefbhand electrode 10 as viewed in the drawing has disposed at theexposed end a screw threaded end portion 16 of reduced diameter anddrilled in the interioran axial bore 18 of small diameter communicatingwith an axial bore 20 extending through that portion of the insulation14 interposed between both electrodes. The bore 20 then communicateswith an axial bore 22 partly extending through the other electrodes 12and terminating in a screw threaded hole 24 larger in diameter than thebores 22 and opening at the exposed end of the electrode 12.

The insulation 14 is preferably of a mixture of powdered mica andpowdered glass. Such a mixture is placed in a mold (not shown) havingboth electrodes 10 and 12 disposed in place therein and heated andmolded under a pressure into the desired unitary structure.

As shown, a screw threaded plug 26 is screw threaded into the hole 24 inthe electrode 12 and has a hole 28 greater in diameter than the bore 22and forming a hermetically sealed compartment generally designated bythe reference numeral 30 along with the axial bores 18, 20 and 22. Theplug 26 projects beyond the exposed end of the electrode 12. Thecompartment 30 has a pressure relief member 32 disposed at the bottom ofthe larger hole 28 and a current limiting material 34 filling theremaining portion of the compartment 30.

The current limiting material 34 is composed of a low melting pointmetal normally put in the form of a liquid or a solid electrically goodconductive such as Na, K, NaK, Hg or the like. The material isresponsive to a high current in excess of a predetermined magnitude tobe evaporated. The pressure relief member 32 may be preferably a bellowsfilled with any suitable inert gas such as argon.

The device thus formed can be connected to the associated circuit (notshown) by having both the screw threaded end portion 16 fitted into aconductor 36 which is, in turn, fastened to the electrode 10 by a nut 38and the plug 26 fitted into a conductor 40 that is, in turn, fastened tothe electrode 12 by a nut 42. Then conductors 34 and 38 are connectedacross the circuit.

The arrangement thus far described is operated as follows: Upon theoccurrence of a short circuit current, it flows from the conductor 34for example, through the electrode 10, at least one portion of thecurrent limiting material 32 and the electrode 12 and thence to theconductor 38 to evaporate at least partly of the current limitingmaterial by means of Joule heat generated therein. The evaporatedmaterial 34 is very high in vapor pressure and presents a highresistance to the shorted circuit (not shown) to limit the current.After the current limiting'operation has been performed, the evaporatedmaterial 34 is rapidly cooled to be self-restored to its original liquidor solid state thereby to be ready for the succeeding operation.

Upon evaporating the current limiting material 34, the pressure in thecompartment 30 is abruptly increased but it is absorbed through thecompression of the pressure relief member 32. However, the pressure inthe compartment 30 can be frequently increased too abruptly to beabsorbed by the pressure relief member 32. If the plug 26 would not beprovided with the hole 28 as in the conventional devices, then the plugwill have axially exerted thereon a pressure sufficient to be blown awayfrom the electrode 12.

According to the principles of the invention, however, such a pressureis also radially exerted on the inner periphery of the hole 28 in theplug 26 to reinforce the engagement of the screw threaded plug 26 withthe screw threaded hole 28. This ensures that the plug 26 is preventedfrom being released from the hole 24 by means of the axial pressureexerted thereon due to an increase in pressure within the compartment30.

While the invention has been illustrated and described in conjunctionwith a single preferred embodiment thereof it is to be understood thatvarious changes and modifications may be resorted to without departingfrom the spirit and scope of the invention. For example, instead of thebellows, the pressure relief member 32 may be a hollow bulb of anysuitable synthetic rubber or a slide member slidably fitted into thehole 28 and normally biased toward the bore 22 by means of a gas or aspring.

What is claimed is:

1. A current limiting device comprising, in combination, a pair ofelectrodes disposed in spaced opposite relationship, an electricinsulation for insulatingly interconnecting said electrodes into aunitary structure, a compartment formed within after which it isrestored.

2. A current limiting device as claimed in claim 1 comprising a pressurerelief member disposed in said hole formed in said plug.

1. A current limiting device comprising, in combination, a pair ofelectrodes disposed in spaced opposite relationship, an electricinsulation for insulatingly interconnecting said electrodes into aunitary structure, a compartment formed within said electrodes and saidinsulation, a screw threaded plug screw threaded into one of saidelectrodes and provided with a hole forming a part of said compartment,and a current limiting material filling said compartment and responsiveto the occurrence of a current in excess of a predetermined magnitude tobe evaporated to perform the current limiting operation after which itis restored.
 2. A current limiting device as claimed in claim 1comprising a pressure relief member disposed in said hole formed in saidplug.